For a hydrogen generator of producing a gas containing a large amount of hydrogen from an organic raw material such as natural gas and naphtha, a steam reforming method of applying heat from an external source to cause a reaction between a raw material and water on a reforming catalyst is often used. As the reforming catalyst, a catalyst having an Ni based or Ru based catalyst carried on a carrier such as alumina is generally used.
The organic material often contains a sulfur containing component such as an odorant component of urban gas or a sulfur component existing originally in crude oil or the like, for example. The sulfur component is essentially a catalytic poison component for many catalysts. Particularly, in the steam reforming method in which a reaction proceeds under a reduced condition, a sulfur component tends to remain on a catalyst, and its catalyst toxicity becomes stronger to cause a decrease in steam reforming reactivity.
Thus, a desulfurization unit to remove a sulfur component in a raw material in advance is often used in combination with the hydrogen generator. For removing an odorant component of a gas such as urban gas, an absorbent material such as zeolite can be used. In addition, for industrial applications, a hydrodesulfurization method is used in which hydrogen is added to the raw material, the sulfur component is made to react into hydrogen sulfide using a catalyst for hydrogenation such as an Mo based catalyst, and then the hydrogen sulfide is removed with an absorbent such as zinc oxide.
Next, there is an autothermal method in which a part of hydrocarbon based fuel is oxidized, and heat generated by the oxidation is used to cause a reaction between the remaining fuel and water, as a typical method of producing a gas containing a large amount of hydrogen from a hydrocarbon based fuel, in addition to the steam reforming method. When comparing the steam reforming method with the autothermal method, one has features which the other does not have.
In the steam reforming method, a produced gas is balanced with steam, hydrogen, carbon dioxide and carbon monoxide because no gas is introduced from the outside. In the autothermal method, the produced gas is balanced with steam, hydrogen, carbon dioxide, carbon monoxide and nitrogen if air is used as an oxidizing gas. Therefore, the steam reforming method can provide a gas containing a higher concentration of hydrogen from a viewpoint of hydrogen concentration.
In addition, the hydrocarbon based fuel contains a sulfur containing component such as an odorant component of urban gas or a sulfur component existing originally in crude oil or the like, for example. The sulfur component is essentially a catalytic poison component for many catalysts. Under a reduced condition, in particular, the sulfur component tends to remain on a catalyst. The steam reforming method essentially allows the reaction to proceed under a reduced condition, resulting in stronger catalytic poison toxicity. On the other hand, the autothermal method alleviates a decrease in catalytic activity due to the introduction of oxidation gas, and is thus more advantageous in terms of sulfur resistance.
An invention has been devised such that the content of sulfur component in the raw material is reduced to 0.1 ppb or smaller in advance using a copper-zinc based desulfurizing agent as in Japanese Patent Publication No. 2765950, in order to take advantage of the feature of the steam reforming method providing a high concentration of hydrogen.
In addition, a catalyst having an Ni or Ru based catalyst carried on a carrier such as alumina is used in a reformer to perform steam reforming as a reforming catalyst for use in the steam reforming method, but the Ni based catalyst exhibiting its capability under a reduced condition suffers a decrease in catalytic activity when oxidized. Also, the Ru based catalyst is volatile under an oxidized condition at a high temperature and thus suffers a decrease in activity. Nevertheless, during normal operation, many of reforming catalysts are used under a reduced condition in the presence of produced hydrogen gas, and therefore they hardly suffer a decrease in catalytic activity due to oxidation.
However, at the time of starting operation and stopping operation, air is introduced from the outside of the apparatus, and those catalysts may be oxidized depending on starting and stopping conditions. Therefore, at the time of starting or stopping operation, an operation for replacing residual gases with an inert gas such as nitrogen so that the catalyst is not oxidized is often used in combination.
In addition, if a combustible gas such as hydrogen is entrained in the apparatus at the time of stopping operation, residual gases are generally replaced with the inert gas because the presence of the combustible gas is not desirable in terms of apparatus safety.
The Ru based reforming catalyst has essentially low resistance to sulfur. Thus, the desulfurization method using the above-mentioned method using an absorbent material or the hydrodesulfurization causes a decrease in catalytic activity due to the presence of a sulfur component not sufficiently removed.
Thus, an invention has been devised such that the content of sulfur component in the raw material is reduced to 0.01 ppb or smaller in advance using a desulfurizing agent as in Japanese Patent Publication No. 2765950. In this method, the catalytic activity can be retained because the content of sulfur component in the raw material is reduced to 0.01 ppb or smaller. However, a method in which the desulfurization characteristic is improved to retain a catalytic activity has many problems such that the configuration of apparatus is complicated; it is difficult to manage the level of desulfurization and so on.
On the other hand, the Ni based reforming catalyst has high resistance to sulfur compared with the Ru based catalyst. Therefore, the reforming catalyst is compatible with the desulfurization method using the above-mentioned method using an adsorbent material or the hydrodesulfurization method. However, this Ni based reforming catalyst exhibits its steam reforming feature under a reduced condition. Thus there is a problem such that the reduced condition must be maintained in the apparatus to prevent a situation in which the catalyst is oxidized, and consequently suffers a decrease in catalytic activity even at the time of stopping the apparatus.
In addition, in the hydrodesulfurization method of removing a sulfur component, hydrogen should be added to the raw material in advance. Provided that the apparatus is operated continuously, the hydrogen can be collected in part from the exit of the apparatus and used. However, hydrogen should be obtained in advance at the time of starting the apparatus. If considering a hydrogen generator as a plant scale apparatus, the necessity to obtain hydrogen does not pose a serious problem. However, if considering a compact-type hydrogen generator, for example a hydrogen generator as a domestic hydrogen source, the necessity to obtain hydrogen poses a serious problem.
Also in the autothermal method, on the other hand, it is essentially desired that the concentration of sulfur in the raw material is lower, and the catalyst reactivity is decreased if the sulfur concentration is high. In addition, there is a problem such that the method cannot prevent a decrease in hydrogen concentration fundamentally.
In addition, in the method in which a desulfurizing agent is used to remove the sulfur containing component in the raw material, the catalyst reactivity is varied depending on the desulfurization condition of the desulfurizing agent, and therefore a method of correctly observing the state of the desulfurizing agent is required. In addition, establishment of a method of determining the poisoning state of the catalyst to recover the activity easily is desired.
In short, the conventional hydrogen generator has as a first problem a problem such that a sulfur containing component contained in the raw material cannot be adequately dealt with.
Next, when the hydrogen generator is continuously operated, an oxidized condition is not created in the apparatus. Therefore, in the case of continuously operating hydrogen generator used for industrial applications or on plant scales, there is little decrease in activity due to oxidation of the catalyst.
When the hydrogen generator is used for domestic applications, however, the apparatus may be started and stopped frequently. If a power generating apparatus is operated for domestic applications, for example, operations to deal with a load change of power are required, and thus the operation of the power generating apparatus is started and stopped frequently. If a fuel cell power generating apparatus is introduced as the power generating apparatus, the hydrogen generator supplying hydrogen serving as a fuel is used in combination. Therefore, the operation of the hydrogen generator should be started and stopped frequently.
For domestic applications, however, it is not easy to obtain an inert gas with which residual gases in the apparatus is replaced in terms of costs and maintenance.
In short, the conventional hydrogen generator has as a second problem a problem such that it is difficult to replace gases in the apparatus effectively and safely at the time of starting and stopping the apparatus.